The demand for high-density and high-speed performance in smaller, more portable electronic devices is ever increasing. Embedded passives comprise a type of Printed Circuit Board (PCB) design that combines better performance in smaller boards with increased functionality by burying resistors or capacitors, i.e. passive devices, in the substrate material itself.
In particular, important performance requirements for making PCBs having embedded resistors operating in the 100 Ohms to 1 mega-Ohm range include a balanced Temperature Coefficient of Resistance [“TCR”] and resistive stability. However, new materials are required since current materials satisfy neither the performance nor fabrication requirements for routine manufacture of embedded passives in PCBs.
Previously, most low TCR resistors consisted of lead- and/or ruthenium-based materials as conductive fillers. Governmental regulations spurred by environmental concerns are eliminating lead from electronic devices, whereas ruthenium is too costly a precious metal for use in high performance resistors.
To compensate current resistors use graphite and metal powders, which are sufficient only for the low resistance range, that is, less than 100 Ohms, and therefore do not achieve the desired resistance operating range. Moreover, those resistors having only graphite as conductive filler tend to have large, negative TCRs in the high resistance range. Neither kind of resistor is satisfactory in performance or material.
Sakai et al (U.S. Pat. No. 4,601,848 A) describe resistor compositions comprising a glass, an inorganic aggregate, carbon black and a reducing metallic powder.
Nishii et al (U.S. Pat. No. 4,732,701 A) describe polymer compositions having positive temperature coefficients comprising a polymer, an electrically conductive powder and a semiconductive inorganic substance.
Chu et al (US 20030111648 A1) describe a resistor composition comprising a polymer, at least one conductive filler and a coupling agent.
The present invention provides compositions comprising titanium nitride or titanium carbide or other nitrides or carbides of group IV elements, an allotrope of carbon, and a polymer binder.